High Energy Electron Irradiated Polystyrene: Free Volume and Thermal Properties Studied by PALS and DSC

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The annihilation of p-Ps is unaffected by the material properties due to its very short lifetime and measures the correctness of the spectrum. Figure 1 shows the variation of the intermediate lifetime and intensity (I₂) components with irradiation dose (KGy) attributed to the annihilation at the crystalline defects or at the interface between crystalline and amorphous domains in the polymer. Figure 2 shows the variations of the o-Ps lifetime component and Intensity (I₃) with irradiation doses (KGy). The behavior of and I₂ in Figure 1 is very similar to that of and I₃ in Figure 2 but the variation in and I₃ is an evidence for the o-Ps being localized at the free volume holes ^[26]. The variation of the most important parameters o-Ps lifetime and intensity (I₃) with irradiation dose (KGy) shows that increases at 50 KGy dose and reaches its lowest value at 75 KGy and as the irradiation dose increases beyond 75 KGy, it again increases. The variation of I₃ shows exactly the opposite behavior of as expected. In Figure 3. the variations of the free volume fraction (V_f)and the mean free volume (V_h) have been plotted against irradiation dose (KGy).

Figure 2. Variation of the o-Ps lifetime component

and Intensity (I₃) with irradiation dose (KGy)

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Figure 3. Variation of the free volume fraction (V_f)and the mean free volume (V_h) with irradiation dose (KGy)

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The size of the free volume hole (V_h) and the free volume fraction (V_f) are calculated by using above three relations. V_h is directly related to and shows the same pattern. But V_f is a function of both and I₃.The value of V_f gives the fraction of free volume in percent (%) not the size. From Figure 3 it is observed that the variation of V_f follows the variation pattern of V_h and reaches its minimum value at the dose 75 KGy.

Irradiation with high energy electron beam promotes two simultaneous processes within the polymer. It produces free radicals and as a result forms new bonds or cross-linking which decrease o-Ps lifetime as well as free volume. The other process is scission or depolymerization which ruptures molecular chains as well as creates new unoccupied spaces (i,e. free volume in polymer) and increases o-Ps lifetime as well as the size of free volumes ^{[10, 19]}. From Figure 2 and Figure 3 it is observed that the first process i.e. cross-linking becomes dominant at dose 75.

Figure 4 shows the variation of specific heat (C_p) of PS with both KGy and causes decrease in o-Ps lifetime and amount of free volume. Both below and beyond 75 KGy, chain scission dominants over cross-linking and results in increase of o-Ps lifetime and amount of free volume. This behavior indicates that most stable structure is achieved at 75KGy e-beam dose.

3.2. DSC Results and Heat Capacity

DSC measures the change in thermal properties and due to microstructural changes of any sample as a function of temperature. The variation of specific heat (C_p) of electron irradiated Polystyrene (including the reference one) with temperature both above and below the glass transition temperature (T_g) are observed. The effect of irradiation dose on C_p is though prominent but on T_g it is not pronounced.

Since the DSC is at constant pressure, heat flow is equivalent to enthalpy changes

In DSC measurement the specific heat of the sample can be determined by comparing it to that of a known sample. In our experiment we have considered sapphire as the standard known sample. The heat that flows into the sample is directly proportional to the specific heat. The heat flow into the sample is given by,

Where dH/dt is the heat flow rate measured in calories/seconds, m is the mass of the sample measured in grams, C_p is the specific heat measured in Calories/gram/^oK, dT/dt is the rate of change of temperature in ^oK/second.

This equation is used in order to derive the final equation which determines the specific heat of the sample given by,

Where C_p′ and m′ are the specific heat and mass of the standard, y and y′ are the ordinate deflections due to the sample and standard respectively measured in centimeters ^[28].

The heat capacity of a substance can be defined as the amount of heat required to change its temperature by unity. A more useful quantity is the specific heat capacity, which is the heat capacity per unit molecular weight of the substance. Heat capacity is one of the most important thermal parameter because its macroscopic behavior can be linked with the microstructure of a substance and hence can be treated as an entity capable of illustrating the physical endeavor of atoms or molecules within a substance to maneuver themselves upon heating ^{[29, 30]}.

Figure 4 shows the variation of specific heat (Cp) of PS with both temperature and irradiation dose. It is found that the glass transition of PS starts in the vicinity of 90-100°C with an endothermic enthalpic relaxation peak almost at the end of the glass transition process. It occurs immediately after the enthalpic relaxation, right after the glass transition process is complete. At a temperature around 115°C – 120°C, disordered PS chains start to order themselves in regular spherulitic structures. Spherulite is the basic higher order structure of crystalline polymers composed of lamellar structures filling in the inner part by the repetition or the re-orientation and branching on the growth in the radial direction.

Figure 4. Variation of specific heat (Cp) of Polystyrene (PS) samples with temperature and irradiation doses (KGy)

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The variations of Cp values have interesting features; it increases with temperature up to the glass transition temperature and then decreases up to a certain temperature (~180°C) and again increases with increase of temperature. It also shows a trend to give another peak at the temperature range 240-250°C due to melting on heating and crystallization on cooling. But PS has a rather bulky pendant phenyl group at regular intervals; steric hindrance makes it difficult to crystallize evenly and extensively. Thus its crystallization peak is not very sharp and is rather diffusively spread. High energy irradiation thus may be thought to set off two primary processes which may lead to structural changes in PS. They are chain scission and cross linking. In effect, these are competitive processes, one leading to a lowering of molecular weight and the other increasing it. Both have a direct bearing on the packing and the crystallization of the polymer. In all probability, the process of chain scission turns out to be a dominating phenomenon. The overall effect of this is a decrease in the chain lengths as well as the molecular weight upon exposure to radiation and it also decreases the value of Cp.

The variation of glass transition temperature (T_g) with irradiation doses are shown in the Table 1. At 50 KGy dose T_gincreases by 1.5°C with respect to the reference sample. The Tg value at 75 KGy and 100 KGy are almost same and lesser than that of 50 KGy. The variation of Tg is negligible, thus it does not give any significant information on the microstructural changes in PS upon e-beam irradiation.

Table 1. Glass Transition Temperature (Tg) at different Irradiation doses (KGy)

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